1. Field of the Invention
The present invention relates to a paint of a shielding coat membrane and a lamp with the shielding coat membrane used for a high luminance discharge lamp of automobiles.
2. Description of the Related Art
Used for a high luminance discharge lamp for headlights of automobiles, it is necessary to form a black shielding coat membrane with an arbitrary shape on a glass surface of the lamp for contrasting light and shade in the projecting area.
FIG. 1 shows a structure of the discharge lamp of the D2R type automobiles. Numeral 1 of FIG. 1 denotes a shielding coat membrane of which position and size are determined by the international standard. Numerals 2a, 2b denote electrodes made of tungsten in which the discharging emission is generated when the electrodes 2a, 2b are applied with an external voltage so that the lamp glows. Numeral 4 denotes an outer tube made of quartz glass. The shielding coat membrane 1 is formed on the surface of the outer tube 4. Numeral 5 denotes an emission tube melted and fixed with the emission tube 5 at a portion 6.
According to the conventional manufacturing process, first the outer tube 4 is melted and fixed with the emission tube 5 with a heating treatment by a gas burner. And the paint for the shielding coat membrane which is made of a black colorant of transition metal elements such as ferric oxide, copper oxide, and cobalt oxide, and also made of a binder of granulated glass is painted on the surface of the lamp. Then the lamp is sintered in the electric furnace at the temperature that the granulated glass is melted so that the desired shielding coat membrane is provided.
As the temperature at the glass surface of the lamp increases when the discharge lamp glows, the temperature of the shielding coat membrane formed on the glass also increases. Especially as for the discharge lamp of the D2R type automobiles, the temperature at the glass surface of the lamp increases up to 700.degree. C., therefore the shielding coat on the glass surface is also exposed under the temperature of 700.degree. C.
For the above reason, it is necessary to sinter the conventional lamp at the temperature more than 700.degree. C.
While the granulated glass is conventionally used for the binder as described, the melting granulated glass does not have a good wettability with the above mentioned black oxide colorant. Thus the granulated glass does not perform as the binder to bind the black oxide colorants one another so that the robust membrane is difficult to be obtained and the cracks sometimes occur thereon.
Therefore it is desired for the shielding coat material which has good membrane strength and no crack.